Friday, 28 August 2015

Revoking access

Access revocation is impossible if you give up everything right away. There's no such thing as decryption that stops working after a certain time. If someone has the keys, the ciphertext and the decryption method, you've given up all control. The only place that access revocation can mean anything is for ongoing services that can't be copied and require a live server to provide. Compare movies to games. If you have a DVD that's encrypted, and it gets decrypted every time you watch it, it can be copied and you can watch it from then on in any format you like, at any time you like, on any device you like, because you've got the decrypted form and it can be copied perfectly. It makes no difference at that point if the DVD publisher puts out an update to their players that says "don't decrypt this movie any more", because the copy is already made.

Conversely, if you need server access to play a certain game, then one day that server won't let you in any more, you can't do anything about it. The previous games you've played don't do you any good. They're over. Backups, news, and social media are like that, because you need access or new content to use them. Your old backups aren't good enough if your provider refuses access. Gradually stagnating news isn't any good except as a historical reference.

Access to books, music and movies, however, is like the first case. Once that genie is out of the bottle, that's it. You can't stuff it back in. If you've got a closed system like the Kindle you can reach out and revoke copies of books, but if the DRM has been broken (which is very different to breaking encryption) and a copy already taken, it doesn't matter.

Key revocation would require the universe to have the property that I can tell you a secret that is only knowable for a specified time, or whose use is, in some way, dependent on a secret I didn't tell you.

Mokalus of Borg

PS - That can't actually happen, as far as I can see.
PPS - Granted, these things don't always make sense to me.